1. Field of the Invention
This invention relates, generally, to a lost wax casting tree and the method of use thereof, in an investment molding process. More particularly, the invention relates to a casting tree which permits forming both white gold and yellow gold portions of a jewelry item, such as a ring, within a single flask, utilizing otherwise standard equipment.
2. Description of the Prior Art
The lost wax/investment molding procedure has found considerable application in dental, jewelry and allied fields where the precise reproduction of an article, as represented by an expendable pattern, is desired.
The process is generally characterized by the formation of a wax or plastic pattern which is embedded in a mixture of refractory investment materials. The resulting pattern is then subjected to heat, in order to drive out moisture from the investment material, to harden it and to melt the wax or plastic from the pattern. Finally, the molten material which is to be cast in the mold is poured into the investment material die and, when the molten material has sufficiently cooled, the die is removed leaving a precise reproduction of the article originally represented by the wax or plastic pattern.
For the purposes of this application, the terminology "wax" or "plastic" is intended to include all those materials which are normally used or are suitable for use in the various lost wax casting processes to which the present invention relates.
The use of a soft material, such as waxes or plastics normally employed in lost wax casting processes, gives considerable leeway to the person preparing the casting as these materials are easily worked. Thus, an artist can sculpt his idea in wax and thereafter have it precisely executed in gold or other suitable material. The ease with which the lost wax techniques can be applied have made them useful for dentists in casting the filling for a cavity or replacing a tooth. Wax can be pressed into the cavity of a tooth, once the cavity has been cleaned, to form an exact duplicate of the cavity. This wax model is then used to form an investment material die wherein gold or other material may be cast to produce a filling for the tooth.
In order to cast molten metal into a prepared mold, a conduit leading to the mold is usually formed. This is accomplished by attaching a piece of fusible material, such as a rod of wax or plastic, to the item being cast. When the wax or plastic is removed a conduit to the mold is formed.
One method of forming the conduit leading to the mold is to attach a rod of the wax or plastic material, known as a sprue, to each wax pattern, and form the casting mold or flask about these individual patterns. The molten gold or other material is then poured into a funnel shaped cavity formed near the surface of the mold, in communication with each sprue or rod. The use of a sprue or sprues is shown, for example, in connection with the formation of various dental castings in U.S. Pat. No. 1,595,338 to Brazda et al.
An improved sprue device which can be adjusted to support various size patterns by bending a U-shaped double sprue to form a wider or narrower "U", as required by the extent of the pattern, is shown in U.S. Pat. No. 2,468,479 to Barishman. This also provides two passages through which molten material can flow to the pattern. U.S. Pat. No. 3,610,317 to Benfield, again shows the use of a sprue or plurality of sprues, each attached to a wax pattern to form conduit to the mold formed by the wax pattern.
A sprue may either be of a fusible material which can be melted from the investment mold, or of a metal which must be withdrawn to permit the wax or plastic used to form the pattern to flow out of the investment mold.
The use of a second sprue which may communicate with one or more of the wax patterns to form a second conduit for the molten material is also shown in Benfield. This second conduit helps to insure complete filling of the pattern especially where there are thin sections in the pattern which might prematurely block the flow of molten material through the main sprue formed conduit. U.S. Pat. No. 3,322,187 to Weissman shows a still further arrangement wherein conduits are formed to a number of patterns by a separate sprue associated with each pattern. U.S. Pat. No. 2,065,977 to Jefferies, although generally concerned with a method of vibrating the investment materials to form a tight mold, again shows the basic technique of the use of sprues to form conduits directly to the wax patterns.
The present invention is intended to be particularly useful in the preparation of parts for forming jewelry and therefore mention is made of U.S. Pat. No. 1,389,315 to Moats. Moats teaches a method whereby a minimum number of different parts can be used to form rings of different sizes. A single inlet conduit which is formed in the mold can be used to pour a single molten material into two molds at the same time.
Although utilizing basically the same technique, it has been found advantageous to cast a larger number of small items in a vertical mold than was previously attainable by the use of individual sprues extending from the opening in the mold, so that a larger number of items are formed from the same casting operation with a minimum of equipment and a minimum of waste of investment molding material. Basically, this is accomplished by forming a "tree" of a fusible material to which is attached the various wax or plastic patterns by means of individual sprues. The result is a single stem which forms a central conduit extending generally vertically into the investment material, with a number of branches extending therefrom to the individual patterns. In general, the tree is formed by first attaching a sprue to the wax or plastic model and then attaching the other end of the sprue to the central stem by the addition of wax or by melting or otherwise. With various modifications in the equipment used or the technique employed, this basic process is shown in U.S. Pat. No. 3,648,760 to Cooper with reference to dental casting; and in U.S. Pat. No. 3,402,755 to Christian, with reference to jewelry articles in general.
It has recently become popular to have jewelry, and in particular rings, formed of both white and yellow gold. This is normally accomplished by separately casting the yellow gold portions and the white gold portions of the jewelry piece and thereafter assembling the portions by soldering or other usual means.
Several practical problems are encountered when this type of jewelry is being made. These problems are associated with the fact that each part must be separately cast and the various parts thereafter matched to each other and assembled. For example, many jewelry items are formed of individualized parts or pieces that are not interchangeable. This requires that some accurate inventory method be used to assure that the right parts come together so that the person assembling them can work effectively. Even when a large number of interchangeable parts are involved, separate casting of each part still requires that there be some inventory method used to insure that the various parts are properly brought together.
Because normally small quantities of jewelry pieces are being made, the cost of a double operation wherein first the white gold pieces and then the yellow pieces are cast, is considerable. In addition, because of the nature of the process, there is considerable waste of investment material in completing a small number of items.
One prior attempt to solve this problem is to first cast the white gold (higher melting material) portion of the ring or other jewelry. The cast white gold portion is then attached to the wax model of the yellow gold (lower melting material) portion to form the combined model which includes the yellow gold mold. When the wax is removed from the investment mold, the white gold portion remains to be fused to the molten gold poured into the mold. Although this solves the problems of inventories of unassociated parts, it introduces unacceptable new problems:
1. There is a considerable waiting period (as much as four hours) for the completion of the first casting step to produce the white gold part, before the combined model can be assembled and thereafter made ready to cast the yellow gold portion to form the complete item. Until the first casting step is completed, the yellow gold portion of the mold is just sitting on the production line, without being worked on, thereby increasing the production time for the complete item and increasing the manufacturing cost.
2. In the curing of the investment material to form a mold, it is baked in an oven. This baking step oxidizes the white gold piece to a black color which is difficult to remove and leaves a white gold piece which never really looks right.
3. The prolonged time that the white gold piece is kept in the oven until curing of the yellow gold portion of the mold causes the white gold piece to become disfigured.
Therefore, the present invention has as a main object to provide a means, and method of using the means, whereby two different materials, and in particular white gold and yellow gold, can be cast in the same flask with sequential pourings of the two molten metals.
It is a related object and feature of the present invention to provide a tree for preparing a tandem mold wherein metals of different melting points can be separately cast.
It is a still related object, feature and advantage of the present invention to provide a mold wherein different parts of a single item may be cast in different materials in the same casting flask.